It is possible to rewrite the information recorded on magneto-optical recording media, which are large in storage capacity and highly reliable. Therefore, magneto-optical recording media begin to be practically used as computer memories etc. In view of an increase in amount of information and downsizing of devices and/or apparatuses, however, higher density recording and reproducing techniques are demanded. Various methods are suggested for producing high density magneto-optical recording media. One of the methods is called the multi-layer recording or multiple recording method for producing a high density magneto-optical recording medium by forming multiple recording layers in a magneto-optical recording medium, and recording information on each layer.
A system for recording multi-valued signals is known as described in, for example, Digests of the 13th Annual Conference on Magnetics in Japan (issued in 1989), page 63 and Japanese Journal of Applied Physics, Vol. 28 (1989) Supplement 28-3 pp. 343-347.
The multi-valued recording system involves stacking (or laminating) a plurality of magnetic layers having different coercive forces, and modulating in multiple levels or stages the strength of a magnetic field applied to the magnetic layers, thereby selectively inverting the magnetization of a specified magnetic layer. It is described that this system makes four-valued recording of signals possible by providing in a recording medium three magnetic layers having different coercive forces.
According to the multi-valued recording system for magneto-optical recording media, however, in order to detect for reproduction the multi-valued signals recorded on a magneto-optical recording medium, these signals have been distinguished by slicing at different levels the signals detected from the medium. Accordingly, it has been impossible to obtain large differences between the signal amplitudes corresponding to the multi-valued states, making it difficult to definitely distinguish two states close or near in signal amplitude. For this reason, the reproduced multi-valued signals have had a low S/N ratio. Therefore, there has been a demand for a reproduction technique for reproducing signals at a high S/N ratio from a magneto-optical recording medium with information recorded at high density.
There is conventionally known no magneto-optical recording medium having a plurality of magnetic layers on each of which information can be recorded and from each of which information can be reproduced independently. A recording medium having such performance may be very effective in recording various types of information correlatively on a single recording medium, or recording and reproducing pieces of channel information simultaneously in parallel.
When it is intended to reproduce information from a recording mark recorded at high density, a problem arises concerning the optical reproducing resolution which depends on the spot diameter of a reproducing light beam. For example, it is impossible to perform reproduction while distinguishing a minute mark having a domain length of 0.15 μm by using a reproducing light beam having a spot diameter of 1 μm. In order to eliminate such restriction on reproducing resolution resulting from the optical spot diameter of a reproducing light beam, an approach has been suggested concerning the magnetically induced super resolution technique (MSR) as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, page 201 (1993). This technique utilizes the occurrence of temperature distribution over a magnetic film in a reproducing light beam spot when a magneto-optical recording medium is irradiated with a reproducing light beam. A magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolution without reducing the actual spot diameter of the reproducing light beam. However, in the case of this technique, since the effective spot diameter is decreased by the magnetic mask, the amount of light which contributes to the reproduction output is decreased, and the reproduction C/N is lowered to that extent. As a result, it is difficult to obtain sufficient C/N.
Japanese Patent Application Laid-Open No. 8-7350 discloses a magneto-optical recording medium comprising a reproducing layer and a recording layer on a substrate. Reproduction can be performed while magnifying the magnetic domains in the recording layer. When the magneto-optical recording medium is subjected to reproduction, an alternating magnetic field is used as a reproducing magnetic field to alternately apply a magnetic field in the direction for magnifying a magnetic domain and a magnetic field in the opposite direction, thereby magnifying and reducing the magnetic domains.
However, the multi-layer recording or multiple recording method involves recording information on or reproducing information from one recording layer with incident light intersecting (or crossing) another recording layer, which absorbs or disperses the light, thus reducing the amount of light contributing to the recording or the reproduction. As a result, it is difficult to obtain a sufficient S/N ratio.
The technique disclosed in Japanese Patent Application Laid-Open No. 8-7350 merely has the effect of improving the quality of signals for only a single recording layer. This technique does not have the densifying effect achieved with a plurality of recording layers laminated or stacked.